1. Field of the Invention:
This invention relates to the inspection of glass articles for the detection of stress in surfaces thereof and is more particularly concerned with apparatus and method for inspecting surface treated glass ophthalmic lenses for the presence or absence of stress patterns prior to dispensing such lenses to the public, especially in cases of the more difficult to detect stress patterns which result from lens strengthening by ion exchange treatment.
2. Discussion of the Prior Art:
In order to enhance the impact resistance of glass ophthalmic lenses so as to safeguard against injury to the eyes of the wearer and yet not require ophthalmic lenses to be of ungainly and/or prohibitive thicknesses and weight, it has become customary to treat the surfaces of these lenses in such manner as to introduce a surface compressive stress therein. This may be accomplished by thermal tempering or chemical ion exchange treatment. Thermally tempered glass is produced by rapidly cooling a glass piece which has been heated to near the softening point. This produces a compressive stress in the surface of the article while the interior of the article is under tensile stress.
The most commonly used process to strengthen ophthalmic lenses by ion exchange utilizes the introduction of a compressive stress at the surface by the substitution of a larger monovalent alkali metal ion for a smaller monovalent alkali metal ion in the surface of the article causing its surface to become more densely packed than its interior.
The terms "surface treatment", "surface treated" and variations thereof as used in this specification and its appended claims are intended to include all surface layers which may be used to strengthen the subject articles, regardless of the precise method by which such layers are formed. Layers of stress are herein considered as being surface treated layers whether produced by thermal tempering, chemical ion exchange or otherwise.
Since neither of the foregoing methods of surface treatment alter the appearance of the treated article sufficiently so that the presence or absence of treatment may be ascertained visually and there is the aforesaid need to determine the presence or absence of treatment whether produced thermally, chemically or otherwise, industry heretofore has been obliged to invest in complex and expensive testing apparatuses or systems especially when testing for the presence or absence of the considerably more difficult to detect surface stresses produced by chemical treatment (ion exchange). Exemplary of such systems are those of U.S. Pat. Nos. 3,746,450 and 3,807,867.
The alternative to employing the aforesaid relatively complicated and expensive detecting apparatuses being relatively unreliable systems for visually observing birefringence of relatively small portions only of lenses, has left the industry with a need for greater simplification, economy and reliability in method of testing ophthalmic lenses for the presence or absence of layers of surface compression, i.e. the presence or absence of enhanced impact resistance.
Accordingly, a principal object of the present invention is to provide apparatus and method for reliably positively detecting the presence or absence of thin layers of stress in ophthalmic lenses and more particularly to make possible the detection of treatment having been performed by ion exchange with positive assurance of either its presence or absence.